I. HISTORICAL SUMMARY
In 1827 the English botanist, Robert Brown, first made mention of the fact that minute particles of dead matter in suspension in liquids can be seen in a high-power microscope to be endowed with irregular wiggling motions which strongly suggest “life.”[70] Although this phenomenon was studied by numerous observers and became known as the phenomenon of the Brownian movements, it remained wholly unexplained for just fifty years. The first man to suggest that these motions were due to the continual bombardment to which these particles are subjected because of the motion of thermal agitation of the molecules of the surrounding medium was the Belgian Carbonelle, whose work was first published by his collaborator, Thirion, in 1880,[71] although three years earlier Delsaulx[72] had given expression to views of this sort but had credited Carbonelle with priority in adopting them. In 1881 Bodoszewski[73] studied the Brownian movements of smoke particles and other suspensions in air and saw in them “an approximate image of the movements of the gas molecules as postulated by the kinetic theory of gases.” Others, notably Gouy,[74] urged during the next twenty years the same interpretation, but it was not until 1905 that a way was found to subject the hypothesis to a quantitative test. Such a test became possible through the brilliant theoretical work of Einstein[75] of Bern, Switzerland, who, starting merely with the assumption that the mean kinetic energy of agitation of a particle suspended in a fluid medium must be the same as the mean kinetic energy of agitation of a gas molecule at the same temperature, developed by unimpeachable analysis an expression for the mean distance through which such a particle should drift in a given time through a given medium because of this motion of agitation. This distance could be directly observed and compared with the theoretical value. Thus, suppose one of the wiggling particles is observed in a microscope and its position noted on a scale in the eyepiece at a particular instant, then noted again at the end of
(for example, 10) seconds, and the displacement
in that time along one particular axis recorded. Suppose a large number of such displacements
in intervals all of length
are observed, each one of them squared, and the mean of these squares taken and denoted by